The cycle of water in which the seawater evaporates and becomes cloud and then rain, and rainwater soaks into the ground and flows as groundwater or rivers, and the water is used by people in various ways and then returns to the sea.
Especially in urban areas, the natural water circulation route is greatly influenced by waterworks, sewer and other water supply and drainage facilities, and the term is used as a water circulation including both the natural and artificial systems.
An act of preventing rainwater from flowing directly into rivers and sewers. It reduces the flood burden on downstream rivers and so on. It also has an effect for the groundwater to be recharged.
Water Diffusion Method
A method of infiltrating rainwater from the ground surface or the shallow underground into the ground through the unsaturated zone of the soil.
Outflow Control Facilities
Facilities that are installed for the purpose of controlling rainwater outflow, and are divided into storage facilities and infiltration facilities. Storage facilities are categorized into off-site storage and on-site storage, and infiltration facilities are categorized into water diffusion method and well method.
Facilities that infiltrate rainwater by water diffusion method and include osmotic box, osmotic trench, osmotic gutter, water permeable (plate) pavement etc.
A facility in which the surrounding of an osmotic box is filled with crushed stone. It infiltrates collected rainwater from the sides and the bottom into the ground.
A facility in which the surrounding of an osmotic pipe is filled with crushed stone. The pipe is connected to an osmotic box, and leads rainwater in the osmotic box and infiltrates it from the sides and the bottom into the ground.
A facility in which the surrounding of an osmotic gutter is filled with crushed stone. It infiltrates rainwater from the sides and the bottom into the ground.
Water Permeable Pavement
Pavement that permeates rainwater directly into water-permeable pavement and then permeates rainwater into the ground using the permeability of the roadbed. Outflow control function by water-storing function of pavement can also be expected.
Infiltration Capacity at Final Value
Infiltration capacity at a point when it becomes stable after water injection is continued in on-site percolation test by fixed level water method.
The analysis of the infiltration capacity at final value generally uses the Kostiakoff infiltration formula (Ministry of Land, Infrastructure, Transport and Tourism, Public Works Research Institute, Material No. 2126). This method deduces an approximate formula from the measured value and calculates the approximate value for each elapsed time. When this approximate value is plotted on a graph, it forms an approximate curve, and the area indicated in the graph shows the infiltration capacity at final value.
Water Permeable Plate
Pavement that has function to infiltrate rainwater into the ground through water permeable concrete plates and joints. It has the same infiltration principle as water permeable pavement.
Permeability Coefficient (cm/sec）
An indicator of the flow velocity of water in porous material.
A coefficient that is gained through ”PWRI method” (analysis method devised by Public Works Research Institute, Ministry of Land, Infrastructure, Transport and Tourism). It shows the infiltration capacity per fixed area on a permeable surface, water head, and hour whereas permeability coefficient shows the flow velocity.
Impermeable stratum (Water-bearing stratum)
A stratum unit through which groundwater hardly penetrates or does not penetrate.
Water that exists below the groundwater table, fills the gaps between strata and flows under the influence of gravity.
A type of unconfined groundwater that is isolated from groundwater body below by an unsaturated zone. Its water table is called perched water table.
SS (Suspended Substance)
Fine particles that are suspended in water (mg/l) and also suspended in the air for a long time (such as fine dusts). In water, it can be separated by filtration or centrifugation. In rivers it stays away from the riverbed for a long time because of turbulence.
Unit Infiltration Capacity
Infiltration capacity per unit facility, for example per osmotic box, or per meter on osmotic trench. It is deduced by on-site percolation test and permeability coefficient.
A safety factor when a decrease in infiltration capacity due to clogging or groundwater table is taken into consideration.
Unit Design Infiltration Capacity
Infiltration capacity of a unit facility when standard infiltration capacity is calculated from unit infiltration capacity and a decrease in infiltration capacity due to clogging etc. is taken into consideration (applied to designed value).
the permeation capacity decline due to clogging or the like in the standard permeation amount obtained from the unit penetration amount. (Applicable to design value)
Osmotic box, Street osmotic box
㎥/hr･box (per unit facility）
Osmotic trench, osmotic gutter
㎥/hr･m (per unit facility)
Water permeable (plate) pavement, water permeable block
㎥/hr･㎡ (per unit facility）
The water depth in an infiltration facility which is used for calculating the unit design infiltration capacity (In principle, replacement levee crown)
The ratio between the apparent volume of the filler such as crushed stone and the remaining volume (void) after subtracting the actual volume of the filler from the apparent volume.
The amount stored in the inner space of an osmotic box etc. and the void between the filler. (Generally, the porosity of the filler is set at 30%, but the spread sand is excluded from the storage amount.)